Many devices and systems include various numbers and types of sensors that perform various monitoring and/or control functions. Advancements in micromachining and other micro fabrication techniques and associated processes have enabled manufacture of a wide variety of microelectromechanical systems (MEMS) devices. In recent years, many of the sensors that are used to perform monitoring and/or control functions have been implemented into MEMS devices.
One particular type of MEMS sensor that is used in various applications is an accelerometer. Typically, a MEMS accelerometer includes, among other component parts, a movable element, also referred to as a proof mass. The proof mass is resiliently suspended by one or more suspension springs such that it moves when the MEMS accelerometer experiences acceleration. The motion of the proof mass may then be converted into an electrical signal having a parameter magnitude (e.g., voltage, current, frequency, etc.) that is proportional to the acceleration.
In some instances, a MEMS accelerometer may experience harsh accelerations or a relatively high force. In such an instance, the proof mass can move beyond a desired distance. Such, movement can potentially damage the MEMS accelerometer. Additionally, the MEMS accelerometer can exhibit unstable behavior if the proof mass, sense electrodes, and/or other portions of the MEMS accelerometer travel too far when a voltage is applied to the MEMS device. Accordingly, many MEMS accelerometers include one or more types of distance limiters, typically referred to as over-travel stops or travel stops. These over-travel stops are arranged to limit the movement of the proof mass and/or other portions of the MEMS accelerometer.
MEMS accelerometers typically have requirements, or specifications, for overload performance. These requirements place tight restrictions on the over-travel stop structure. That is, the over-travel stop must allow the proof mass to travel a specific distance with little variation in accuracy over a relatively large temperature range.
FIG. 1 shows a top view of a prior art accelerometer 20 having over-travel stops 22. Accelerometer 20 includes a proof mass 24 suspended above and anchored to an underlying substrate 26 via one or more proof mass anchors 28. More particularly, one or more compliant members 30, or springs, interconnect proof mass 24 with proof mass anchors 28. Proof mass 24 includes a number of movable fingers, or movable electrodes 32. Fixed electrodes 34, which may be some combination of sense electrode and/or actuator electrodes, are positioned between pairs of movable electrodes 32, and are formed on or otherwise attached to substrate 26. The horizontal and vertical elements of the illustrated proof mass 24 are represented by a single width lines for simplicity of illustration. However, it should be understood that in actuality these horizontal and vertical elements of proof mass 24 have an actual thickness which could alternatively be represented by a double line.
Accelerometer 20 represents a typical single axis accelerometer. Accordingly, compliant members 30 enable movement of proof mass 24 when accelerometer 20 experiences acceleration in an x-direction 36 substantially parallel to a plane of substrate 26. Movement of proof mass 24 alters capacitances 38 between movable and fixed electrodes 32 and 34 used to determine differential or relative capacitance indicative of the acceleration. It should be understood that physical capacitor structures are not present between movable and fixed electrodes 32 and 34. Rather, capacitor symbols 38 are shown to represent the changing capacitances between movable and fixed electrodes 32 and 34. Over-travel stops 22 limit movement of proof mass 24 when accelerometer 20 experiences harsh or excessive acceleration in x-direction 36 to prevent damage to proof mass 24, sense electrodes 32, 34, and/or other portions of accelerometer 20.
Typically, over-travel stops 22 are anchored, or attached, to substrate 26 at a location convenient to over-travel stops 22. It should be noted that when proof mass 24 is not subjected to acceleration in x-direction 36, a stop gap 40 is present between over-travel stops 22 and a periphery 42 of proof mass 24. Stop gap 40 defines the distance that proof mass 24 is allowed to travel, or move, until it hits one or more of over-travel stops 22. Unfortunately, significant inconsistencies in a width 44 of stop gap 40 have been detected over varying temperatures of accelerometer 20. These inconsistencies in width 44 of stop gap 40 can degrade overload performance of accelerometer 20 and/or can result in loss of accuracy at overload conditions.
Accordingly, there is a need for an improved MEMS accelerometer that is not prone to damage resulting from impacts involving the functional components and is highly accurate over various operational temperatures.